A neutral atom, molecule, or insulating body can respond to an external electric field even when its total charge is zero. The field can shift positive and negative charge slightly relative to one another, creating an induced electric dipole.

For many isotropic particles in sufficiently weak fields, the induced dipole moment is approximately proportional to the local field:

$$\boxed{\mathbf p_{\rm ind}=\alpha_E\mathbf E},$$

where $\alpha_E$ is the electric polarizability. A larger polarizability means the charge distribution is easier to distort.

Polarization of matter

A material can become polarized in two common ways:

• an external field induces dipoles in constituents that had no permanent dipole;
• the field partially aligns constituents that already possess permanent dipole moments.

The material may remain neutral overall while positive and negative bound charge become slightly separated. Polarization is therefore internal charge separation or dipole alignment, not necessarily transfer of net charge into the material.

Why neutral matter can be attracted

In a uniform electric field, the opposite forces on a small induced dipole cancel translationally. In a nonuniform field, however, one side of the dipole lies in a stronger field than the other. The two force magnitudes are then unequal.

For ordinary positive polarizability, an induced dipole generally experiences a net force toward the region of stronger field.

This explains why a charged object can attract a neutral lightweight object. The neutral object becomes polarized; the attraction of the nearer induced opposite charge is stronger than the repulsion of the farther like charge.

Electrostatic attraction therefore does not prove that the attracted object initially carried net charge.

Conductors and insulators

Conductors and insulators respond differently to an applied field.

In a conductor, mobile charge can move over macroscopic distances through the object and redistribute on its surface.

In an insulator, charge is much more strongly bound. Polarization usually comes from small internal displacements of positive and negative charge or from reorientation of molecular dipoles.

Both are electric-field responses, but conductor induction and dielectric polarization are distinct physical mechanisms.

Worked example

Suppose a polarizable particle is in a field

$$E=2.0\times10^5,\mathrm{N/C}$$

and has

$$\alpha_E=6.0\times10^{-40},\mathrm{C,m^2/V}.$$

Its induced dipole moment magnitude is

$$p_{\rm ind}=\alpha_EE =(6.0\times10^{-40})(2.0\times10^5) =1.2\times10^{-34},\mathrm{C,m}.$$

If the field is removed and the particle has no permanent dipole, this induced moment disappears in the simple linear model.

Induced dipoles provide the microscopic bridge between electric fields and the macroscopic polarization of dielectric matter.